Thanks for your quick reply!
OK, I found it. I thought it was a "tile"-property because each of my objects on the layer has a different "anchor"-point and I wanted to specify that point for each tile.
The option does not allow individual pixel values, but thats fine. With the "bottom" setting I can place the objects and when loading the map in the game I can calculate the "correct" position.
Thanks again and keep up the great work!